1. Field of the Invention
The present invention relates to a glass composition having both of a low melting point and a practical chemical resistance and a magnetic head employing it.
2. Description of the Related Art
A magnetic head has employed as a gap forming material, a spacing filling material, a bonding material or as an insulating material a glass with a low melting point containing some components such as silicon dioxide (SiO.sub.2), lead oxide (PbO), sodium oxide (Na.sub.2 O), diboron trioxide (B.sub.2 O.sub.3) or the like.
However, the above composition involves inconsistent relationship between a melting point of a glass and a chemical resistance thereof in which, when one is dominated, the other is deteriorated. Specifically, in order to set a melting point of this kind of glass lower, a large amount of PbO and alkali metal oxide are contained in the glass, which deteriorates the chemical resistance thereof. On the other hand, in order to improve the chemical resistance of the glass, a large amount of SiO.sub.2 are contained therein, which increases the melting point thereof.
Specifically, in general, a large number of magnetic-head materials such as a magnetic-core material or the like have characteristics in which its magnetic characteristic is deteriorated at high temperature. In a magnetic head manufacturing process, a glass is molten and bonded at a temperature at which a normal viscosity thereof becomes 10.sup.3 pa.s (hereinafter referred to as a bonding point) and then soaked in a grinding fluid or the like. Therefore, when the above glass composition is employed, if priority is given to fabrication of a glass having a lower melting point in order to prevent the magnetic characteristic of the magnetic head from being deteriorated, then a glass portion of the magnetic head is chemically damaged in the manufacturing process thereof. If on the other hand priority is given to fabrication of the glass having the chemical resistance, then the magnetic head cannot have a high magnetic characteristic required for a magnetic head.
Specifically, since a melt-bonding glass employed as a bonding material or the like in the magnetic-head manufacturing process is soaked in a grinding fluid and a washing solution both having alkalinity (about PH9 to 11) when a bonded block is ground and processed, the melt-bonding glass must have excellent chemical resistance and particularly excellent alkali resistance. For this end, only employment of the glass having a melt-bonding temperature of 700.degree. C. or higher could provide a result satisfying both of the above requirements.
In order to solve the above problems, the assignee of the present invention proposed in Japanese patent publication No. 315037/1994 a glass composition having both of the low melting point and the chemical resistance and a magnetic head which realized improvement of a quality thereof by using the glass composition.
The above glass composition contains PbO of 60 to 65 weight %, SiO.sub.2 of 20 to 26 weight %, B.sub.2 O.sub.3 of 2 to 10 weight %, tellurium dioxide (TeO.sub.2) of 3 to 8 weigh %, Na.sub.2 O of 0 to 5 weight %, and zirconium dioxide (ZrO.sub.2) of 1 to 3 weight % and has the bonding temperature of 580.degree. C. to 650.degree. C. In particular, the glass composition contains TeO.sub.2 and ZrO.sub.2 and hence has a lower melting point and an improved chemical resistance.
However, if a lower melting point of a glass is required, the above glass composition encounters the problems. Especially, if the glass having the melt-bonding temperature of about 580.degree. C. to 650.degree. C. is selected as a melt-bonding glass used in the magnetic-head manufacturing process using an amorphous metal as described above, employment of such glass enhances crystallization of the amorphous metal, which deteriorates the magnetic characteristic of the magnetic head. Therefore, in order to keep the magnetic characteristic of the amorphous metal effective, it is necessary to bond the amorphous metal with a glass capable of being bonded thereto at a temperature which is as low as possible as compared with a crystallization temperature of the amorphous metal.
Moreover, when the magnetic head is assembled through two or more glass bonding processes, a glass employed in the second and succeeding glass melt-bonding processes must be bonded at a temperature lower than a glass transition point of a glass used in the first glass melt-bonding process. If the glass employed in the second glass melt-bonding process is not bonded at such temperature, then a melt-bonded portion bonded in the first glass bonding process is molten or the like again, which prevents the magnetic head from having a predetermined dimension. Therefore, the glass employed in the second and succeeding glass melt-bonding processes is required to have the melt-bonding point of about 550.degree. C. or smaller, and hence the glass composition having the melt-bonding temperature ranging substantially from 580.degree. C. to 650.degree. C. cannot be employed in the second and succeeding glass melt-bonding processes.
In the general magnetic-head manufacturing process, a melt-bonding glass used for the bonding is usually melt-bonded at a temperature at which its viscosity is about 10.sup.3 Pa.s as described above, and soaked in the grinding fluid and the washing solution in the succeeding processing processes. Consequently, the above glass composition is inevitably involved in the problem that if priority is given to the lower melting point of the glass composition, then the glass portion is chemically damaged in the manufacturing process and, contrary, if priority is given to the chemical resistance thereof and hence the melting point thereof is set higher, then the magnetic characteristic of the magnetic core is deteriorated or the glass melt-bonded in the previous process is molten, thereby a dimension accuracy of the magnetic head becoming unsatisfactory. In particular, since the above melt-bonding glass employed in the magnetic head is required to have a remarkably low melt-bonding temperature, when the glass having the low melting point and the above composition is employed, the glass starts being dissolved in the grinding fluid or the like in the magnetic-head manufacturing or working process, which causes a considerable bump on the glass and cracks the glass. Thus, it frequently becomes difficult to form a shape of the magnetic head.
Specifically, since, when a head block is worked or processed after the glass is melt-bonded, the head block is soaked in the grinding fluid and the washing solution having alkalinity (about PH9 to PH11), the bonding glass employed in the magnetic head must have excellent chemical resistance and particularly the excellent alkali resistance. For this end, the proposed glass composition described above must contain the glass having the melt-bonding temperature of about 580.degree. C. or higher. However, in order to give priority to the magnetic characteristic and the dimension accuracy of the magnetic head, the magnetic head inevitably employs the melt-bonding glass having the low melt-bonding temperature lower than 550.degree. C. Employment of such magnetic head provides unsatisfactory yield of the magnetic head upon the processing and further provides unsatisfactory reliability.